Information regarding respiratory function of a living organism is important in the field of medicine. Respiratory function provides a measure of how efficiently air is moved through the respiratory system, and thus provides important clinical information for the diagnosis and treatment of many respiratory conditions and diseases. Some examples of these conditions are chronic obstructive pulmonary disease (COPD), asthma, and emphysema. In addition, respiratory function measurements allow medical practitioners to observe effects of a bronchodilator or long-term treatments for COPD, or conversely, the airway responses to a bronchoconstrictor challenge for assessment of airway reactivity.
Respiratory function testing includes mechanical function tests which typically compare the effort or driving pressure put forth by the organisms to some quantifiable outcome, such as the output of flow or minute ventilation. Lung function tests differ based on how these inputs and outputs are assessed. Examples of inputs to the respiratory system that are measured, include diaphragmatic electromyographic activity, changes in thoracic esophageal pressure or pleural pressure, changes in airway pressures in ventilated subjects, or noninvasive measures of drive including respiratory inductance plethysmography or impedance plethysmography, and whole body plethysmography. Examples of output measurements include flow, tidal volume, or ventilation measurements using devices that collect flow at the airway opening. In general, the mechanical function of the respiratory system is best described by combining some measure of drive with output. Variables such as resistance and compliance can then be derived to assess the level of airway obstruction or loss of lung elasticity, respectively. This is the basis for classical physiologic modeling of the respiratory system: the comparison of transpulmonary or alveolar pressure changes with flow or tidal volume, carefully assessed in the same time domain with avoidance of phase lag between signals.
Classical physiologic modeling measures total pulmonary resistance, dynamic compliance and related variables. However, the classical physiologic modeling relies on the invasive passage of an esophageal balloon for example, for measuring driving pressure, and flow as a measure of output. An esophageal balloon catheter is positioned in the midthoracic esophagus. Thus, classical physiologic measures are not typically used because of the invasive nature of the esophageal balloon catheter and the difficulty in calibrating the classical system under field conditions.
Lung function tests have evolved with respect to the sensors, recording devices, and analysis techniques used to evaluate input and output. However, a need still exists for the noninvasive determination of lung mechanical function and monitoring in human and animal subjects for clinical and research purposes. In this respect, a number of technologies to measure drive, mentioned above, are available. Devices such as single and double plethysmographs are used to measure drive. In the double chamber plethysmograph, thoracic and nasal flows are recorded as separate signals, whereas in whole body barometric plethysmography, a single signal is recorded that is the net signal from the thoracic and nasal components. The latter is achieved simply on the basis that animals breathe inside a box where pressure changes are the net effects of both components. The aforementioned plethysmographic techniques, completely due to their size and complexity, preclude their use as a portable field test. In addition, these techniques completely enclose the subject, which is objectionable to both humans and animals.
Extending lung function tests to animals has been difficult because technological limitations prevent restraining a conscious animal for prolonged periods of time in devices or chambers such as, without limitation, in plethysmograph chambers. As a result, most lung function studies to date have been limited to animals which are typically anesthetized or conscious animals that provide data with artifacts such as motion artifacts.
Significant challenges remain in conducting lung function tests in conscious animals. The respiratory system of conscious animals, such as canines, is evaluated by physical examination, x-rays, endoscopy, cytology, arterial blood gases, capnography and oximetry. Fluoroscopy can also be used for imaging the respiratory system in small animals, but only if they are sufficiently sedated. Simple measurements of spirometry using a facemask have been conducted in small animals that provide measurements of flow, tidal volume, minute ventilation, and respiratory frequency. These measurements are generally not useful beyond clinical impressions. None of these currently used methods address the mechanical function of the lungs. At best, lung functions can be inferred indirectly such as, for example, lung volume using an x-ray, and dead space based on arterial blood gases or capnography, but these are typically inaccurate measures.
A need still exists for improved systems and methods which provide for measuring respiratory function for health care practitioners, that are portable and which are non-invasive to the living organisms. Further, there is still a need for pulmonary function testing devices for conscious animals.